The present invention relates to optical networking and more particularly to systems and methods for optical amplification.
Available optical fiber bandwidth has been only partially utilized in high capacity optical transmission due to lack of applicable optical amplifier technologies to overcome fiber losses in the wavelength range 1300 nm to 1650 nm. Lumped Raman amplifiers allow the designer to adjust the spectral position and spectral width of the amplification region to optimally account for channel capacity, channel spacing, and transmission fiber type. Lumped Raman amplification also provides important advantages in that the same fiber can provide gain and dispersion compensation simultaneously while maintaining low noise figure in comparison to standard EDFA (Erbium-doped fiber amplifier) technology.
A disadvantage of Raman amplification is poor power conversion efficiency in comparison to EDFAs. For example, a typical lumped Raman amplifier that also provides dispersion compensation may have an efficiency of 2-3% as compared to 8-10% for an EDFA module that incorporates a variable optical attenuator that balances saturation and chromatic dispersion compensation loss. This lower efficiency can be justified only for large bandwidth/high capacity applications where at least 2 EDFAs must be used. For example where both the 1530-1560 nm and 1570-1600 nm bands are used, each band generally requires a two-stage EDFA composed of a preamplifier and a booster. The use of multiple EDFAs raises the cost of that approach to an unacceptable level.
When bandwidths are very wide, (e.g., >50 nm) and the number of channels is also large, input amplifier power will be very large giving rise to saturation difficulties with EDFA technology. However, the saturation advantage of lumped Raman amplifier technology in this situation is limited due to interactions among multiple pump signals that are used to establish the very broad amplification bandwidth. In particular, for bandwidths greater than 60 nm, the combination of pump-to-pump interactions and high input power leads to uncontrollable saturation effects unless effective countermeasures are implemented.
Schemes have been developed to employ pump reuse to provide improved saturation performance, power conversion efficiency, and also noise figure. One approach adds a pump reflector to a counter-propagating pump structure. Another approach adds an optical feedback scheme to a counter-propagating pump structure. These approaches add undesired complexity.
The above-mentioned problems are exacerbated in the context of a metropolitan optical network, e.g., a network where spans range from zero to 80 km. There may be rapid changes in optical network configuration over time due to traffic variations leading to addition and deletion of channels from the channel grid. Also, the number of channels may vary greatly from node to node as channels are added and dropped. Low noise figure, sufficient gain, and appropriate levels of saturation have to be maintained over a wide range of channel counts and system span lengths. EDFA technology can maintain the needed gain and chromatic dispersion compensation over the range of input powers inherent in the variability of channel counts and span length only at the expense of excessive noise figure (e.g., typically from 6 to 12 dB for gain from >20 dB to 12 dB.). If an EDFA is used in a low gain regime the noise figure can reach very high values (e.g., at 5 dB gain, the noise figure can increase up to 25 dB).
Improved systems and methods for optical amplification are needed that can provide appropriate gain over a large bandwidth and broad range of channel counts and input power levels.